1. Field of the Invention
The present invention relates to a combustion apparatus used in heater, water heater, air conditioner or the like, using combustion heat as heat source.
2. Related Art of the Invention
When a catalytic combustion apparatus is operated at a same combustion load factor (combustion amount per unit volume of combustion chamber) as in a flame combustion apparatus, the catalyst temperature exceeds 1200.degree. C., and the heat resistant life of the catalyst is extremely shortened. As means for solving this problem of combustion load factor, for example, as shown in an embodiment of Japanese Patent Application No. 7-316888 in FIG. 1, a combustion system is composed of a first catalytic combustion unit 1 having a heat exchange type, and a second catalytic combustion unit 2 having a honeycomb catalyst provided downstream of the first catalytic combustion unit 1. The fuel is mainly burned in the first catalytic combustion unit 1, and a flame is not formed at its downstream. The first catalytic combustion unit 1 makes use of high heat transfer property of catalytic combustion, and is a heat exchange type catalytic combustion unit having a catalyst 4 provided in a heat receiving fin 3. The water in a cooling route 6 is heated to be warm water in the first catalytic combustion unit and waste heat recovery unit 7. Since the heat receiving fin 3 for heat exchange is directly covered with the catalyst 4, the heat transfer speed of the heat generated in the catalyst to the heat receiving fin is high, so that a combustion system integrated with heat exchanger of small size and high efficiency is realized.
To start combustion in this system, the catalyst must be preheated over the reaction temperature. As this combustion starting method, a method of forming a flame before start of catalytic combustion, and a method of preheating the first catalytic combustion unit 1 second catalytic combustion unit 2 before start of catalytic combustion by electric heater 5 have been proposed.
Such conventional systems, however, involve three problems as explained below.
1. Concerning start of combustion, a method of heating the catalyst to activation temperature by an electric heater, and then feeding fuel is disclosed as a prior art. In this prior art, the electric heater heats all of first catalyst, heat exchanger of first catalyst, and second catalyst, and therefore a large electric power was required for heating the catalyst. A method of preheating by flame is also disclosed, and although the required electric power is small in this method, NOx is contained in the exhaust generated by the flame.
2. Concerning the structure of the heat exchanger of the first catalytic combustion chamber, in the prior art, the heat exchanger fin of the first catalyst projects from the periphery into the center, and therefore the temperature of the first catalyst is not uniform. As a result, the reaction quantity is not same between the center and periphery of the first catalyst, and it leads to various problems such as deterioration of center of the first catalyst due to elevation of temperature, increase of nonreacted amount due to decline of temperature in the periphery, and reaction of the nonreacted portion of the periphery with the periphery of the second catalyst not having heat receiving portion to deteriorate by causing abnormal high temperature.
3. Concerning combustion adjusting width, in the prior art, when the first catalyst drops in temperature to stop reaction when the combustion amount is extremely small, combustion cannot be started again at this point. Hence, the TDR was not sufficiently wide.
In other conventional heating apparatus, in electric heating, when a large current is generated, the cost of the equipment and the running expense increase, whereas, in combustion heating, a large amount of heat can be generated economically, but there are problems of smell of exhaust, in particular, release of smell when igniting, and instability of combustion in small combustion region. Moreover, in the equipment combining combustion and electric heater, the problem of exhaust of combustion is not solved.